1. Field of the Invention
The present invention relates to a semiconductor surge protection device for protecting circuit systems in communication apparatuses, computers, and the like from an overvoltage and overcurrent such as lightning surge and switching surge.
2. Description of the Related Art
Surge protection devices of this type have been achieved by combining pnpn thyristor elements and pn diode elements on a substrate to provide a desired circuit. These techniques are disclosed in U.S. Pat. Nos. 6,104,591, 5,512,784, and 4,644,437.
In a surge protection device having the above arrangement, however, a plurality of parts, i.e., pnpn surge protection elements and pn diode elements are mounted on a substrate. This complicates the mounting process, increases the product size, and makes the product expensive.
Also, the conventional technique comprising a monolithic structure of pnpn thyristor elements and pn diode elements is disclosed in U.S. Pat. No. 6,075,277.
Unfortunately, this technique is applicable only to a circuit configuration, as shown in FIG. 5, in which a pnpn thyristor element Thy4 is inserted between AC terminals of a diode bridge. That is, the technique cannot be applied to a circuit in which the pnpn thyristor Thy4 is inserted between DC terminals of a diode bridge.
Therefore, it is an object of the present invention to provide a monolithic surge protection semiconductor device in which a balance circuit is formed in the same semiconductor substrate.
It is another object of the present invention to provide a surge protection semiconductor device in which diodes are isolated by a double isolation region.
It is a further another object of the present invention to provide a compact, long-life surge protection semiconductor device.